Mimicking Photosynthesis with Electrode-Supported Lipid Nanoassemblies.

The grand scale, ultimate efficiency, and sustainability of natural photosynthesis have inspired generations of researchers in biomimetic light energy utilization. As an essential and ubiquitous component in all photosynthetic machinery, lipids and their assemblies have long been recognized as powerful molecular scaffolds in building artificial photosynthetic systems. Model lipid bilayers, such as black lipid membranes and liposomes (vesicles), have been extensively used to host natural as well as synthetic photo- and redox-active species, thereby enabling key photosynthetic processes, such as energy transfer and photoinduced electron transfer, to be examined in well-defined, natural-like membrane settings. Despite their long history, these lipid models remain highly relevant and still enjoy wide practice today. In this Account, we share with the reader our recent effort of introducing electrode-supported lipid nanoassemblies as new lipid models into photosynthesis biomimicking. This line of research builds off several solid-supported lipid bilayer architectures established relatively recently by workers in membrane biophysics and reveals important new features that match and sometimes exceed what earlier lipid models are capable of offering. Here, our eight-year exploration unfolds in three sections: (1) New photosynthetic mimics based on solid-supported lipid bilayers. This systematic effort has brought three solid-supported bilayers into artificial photosynthesis research: lipid bilayers supported on indium tin oxide electrodes, hybrid bilayers, and tethered lipid bilayers formed on gold. Quantitative on-electrode deposition of various photo- and redox-active agents, including fullerene, Ru(bpy)3 2+ , and porphyrin, is realized via liposomal hosts. Vectorial electron transfer across single lipid-bilayer leaflets is achieved between electron donor/acceptor directionally organized therein, taking advantage of multiple incorporation sites offered by these bilayers as well as their sequential formation on electrodes. Supported on electrodes, these bilayers uniformly afford reliable photocurrent generation and modular system design. (2) Gold-supported hybrid bilayers as a powerful model platform for probing biomembrane-associated photoelectrochemical processes. These hybrid nanostructures consist of one alkanethiol (or substituted alkanethiol) and one lipid monolayer, whose chemical identity and makeup can be separately controlled and modified. Such precise molecular organization and flexible formation, in turn, enable a series of physicochemical parameters key to photosynthetic processes to be explicitly examined and cross-compared. A few such examples, based on donor/acceptor distance and loading, interfacial dipole, and redox level, are included here to illustrate the usefulness and versatility of this system. (3) Mimicking photosynthesis with supercomplexed lipid nanoassemblies. This research effort was motivated to address the low light absorption suffered by single-bilayer based photosynthetic mimics and has yielded a new lipid-based approach to mimicking Nature's way of organizing multiple photosynthetic subunits. Rhodamine and fullerene assembled within these lipid supercomplexes display robust electronic communication. The remarkable possibility of using lipid matrix to further improve photoconversion efficiency is revealed by cholesterol, whose addition triggers exciton formation that promotes faster energy and electron transfer in these lipid nanoassemblies.